Use const in pdf_thumbnail_view.js

[wojtekmaj]

No description provided.

[timvandermeij]

Closing since we decided to convert the entire web folder automatically using ESLint's --fix parameter in ##11450 to both reduce code churn and to be done with the conversion quickly. Thank you for helping out with this though; your patches prompted us to look into ways to automate tasks more, like the linting and const conversion, so that's really positive!